If you don't mind paying more than you would for a generic deep cycle battery go for the biggest Trojan SCS 12V battery you can fit into your space. I use a single SCS200 (Gr 27)which is rated at 115 AH because I depend on my battery when we are dry camping in the Smokies. I don't think I would make it more than two days even with our conservative electrical usage. If we are camping in cold weather we only run the furnace in the morning not during the night. I recharge every morning because I want the battery to not be discharged any more than it has to. We really do need to know more about your electrical consumption to get you a better answer.
Dick